"User & Mailbox are in two different active directory sites" after performing Datacenter Switchover

"User & Mailbox are in two different active directory sites" after performing Datacenter Switchover

Scenario:
A single DAG extended across two active directory Sites(Site A & Site B). 4 mailbox servers in Primary Site & 2 Mailbox servers in DR site having 80 databases in total. As part of standard practice before migrating users to these servers, datacenter switchover was performed to make sure all would be good in case of real disaster scenario. Datacenter switchover was performed successfully & all databases were mounted in the DR servers. But the test mailboxes were not receiving any Emails after switchover. Upon trouble shooting, it was noticed that all those EMails were piling up in submission queue with error message “STOREDRV.Deliver.Exception:WrongServerException.MapiExceptionMailboxInTransit; Failed to process message due to a transient exception with message The user and the mailbox are in different Active Directory sites”

Solution:
The actual root cause of the issue was AD replication. There was no direct site link connectivity in ADSites & Services between the sites where Exhchange servers are deployed which is a best practice to be followed as per Microsoft. Initiating a manual replication could have fixed the issue but we could not do that as AD was taken care by a different team.
But another solution that we found out to fix the issue was to dismount & mount the databases. This caused all the Emails to move from submission queue to Mapi delivery queue & finally to the test mailboxes.

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